Control system for arc voltage of a plasma arc torch

ABSTRACT

A control system for the arc voltage drop in a furnace heated by a plasma arc torch has a voltage transducer providing a signal representative of the arc voltage drop. A function generator, based on the ionizing gas used, alters the signal to provide a constant loop gain. The altered signal is compared to a desired voltage drop and the resulting error signal is used to move the torch towards of away from the furnace.

United States Patent 1191 1111 3,732,351 Schultz 1 1 May 8, 1973 54 CONTROL SYSTEM FOR ARC 3,588,441 6/1971 Dobbs et al. ..219 121 P VOLTAGE OF A PLASMA ARC TORCH [75] Inventor: Ronald G. Schultz, Bay Village,

Ohio

[73] Assignee: United States Steel Corporation,

Pittsburgh, Pa.

[22] Filed: May 18, 1972 [21] Appl. No.: 254,588

52 us. 01 ..13/13, 13/1 [51 Int. Cl. ..H05b 7/00 [58] Field of Search ..l 3/], 9,13; 219/ 121 P [5 6] References Cited UNITED STATES PATENTS 3,520,977 7/1970 Petersen et al. ..l3/l

Primary Examiner-Roy N. Envall, Jr. Attorney-Rea C. Helm A control system for the arc voltage drop in a furnace heated by a plasma arc torch has a voltage transducer providing a signal representative of the arc voltage drop. A function generator, based on the ionizing gas ABSTRACT used, alters the signal to provide a constant loop gain;

The altered signal is compared to a desired voltage drop and the resulting error signal is used to move the torch towards of away from the furnace.

5 Claims, 1 Drawing Figure POWER SOURCE VOL T465 TRANSDUCER {46 POSITION SET POINT CONTROLLER CONTROL SYSTEM FOR ARC VOLTAGE OF A PLASMA ARC TORCH This invention relates to a system for controlling the arc voltage drop in a plasma arc fumace, and more particularly, to a system for automatically controlling the arc voltage drop by controlling the arc gap length.

Plasma arc furnaces are used to obtain a high heating rate such as may be required in the melting of a steel ingot. The temperature of the plasma arc torch is dependent upon the ionizing gas used, the gas flow rate, the voltage drop across the arc, the material being used and the current to the torch. The interaction of these factors, mostly in a non-linear fashion, along with torch design and furnaceconfiguration produces the desired heating rate. The voltage drop across the arc influences the arc temperature, the arc current, and the power supplied to the arc. These factors coupled with the ionizing gas in the plasma arc and the material being heated determine the heating rate. The are voltage drop should remain relatively constant at a predetermined value when disturbances enter the system, so that the arc temperature and heating rate can be maintained at uniform values. Once an ,arc is established the voltage across the arc is proportional to the length of the arc. Any change in an arc length, such as that caused by a movement of the material being heated, changes the arc voltage drop thereby affecting a change in torch current. The arc voltage relation to arc length with a constant current is also afiected by the gas or combination of gases used with the torch.

It is therefore an object of my invention to provide apparatus for controlling the arc voltage drop of a plasma arc torch by varying thetorch arc length.

Another object is to provide apparatus for controlling the arc voltage drop for a variety of ionizing gases.

Still another object is to provide apparatus for controlling the arc voltage drop by varying the torch arc length in a non-linear fashion. 7 g

These and other objects will become more apparent after referring to the following specification and drawing in which the single FIGURE is a schematic view of my invention. i a l Referring now to the drawing, reference numeral 2 indicates a furnace, as for example, for remelting a steel ingot. A plasma arc torch 4 heats fumace 2. Torch 4 has a tiltable mount 6 which moves along track 8 to move torch 4 towards and away from furnace 2. Additional torch adjustment includes a pivot 10 to provide vertical adjustment. Torch 4 has an electrode 12 connected to a power cable 14. An ionizing gas is introduced to the torch through a line l6. Cooling water from a source 18 enters a cooling jacket 20 through a line 22 and exits through a discharge line 24. The parts thus far described relate to a conventional plasma arc torch used to heat a furnace although there may be more than a single torch surrounding furnace 2 and there may be more than a single jet of gas within torch 4. g

A suitable power source 26 connects to torch 4 through cable 14. Power source 26 is also connected to a voltage transducer 28. Transducer 28 is connected to furnace 2 through one side of a single pole doublethrow switch 30 to provide an output 32 which is proportional to the arc drop voltage. Transducer 28 may be a digital voltmeter or a standard recorder through outputs of function generators 34A, 34B and 34C are connected in parallel through switches 36A, 36B and 36C, respectively, to an output line 38. Output 38 is connected to a set point 40, such as a summing amplifier, which has connected to it a desired arc drop voltage set potentiometer 42 and an output line 44. Output 44 is connected to an input of a position set point controller 46. An output 48 of controller 46 is connected to an input of a set point 50, such as a summing amplifier, which has an output 52 connected to an input ,of a position controller 54. Controller 54 has an output 56 connected to a position servo 58 which drives mount 6 back and forth along track 8. Mount travel limiters 60 and 62 provide output signals 64 and 66, respectively, to position servo 58. A position indicator 68 is connected to mount 6 and has an output 70 proportional to the location of mount 6 along track 8. Output 70 is connected to an input of set point 50 and to an input of a function generator 72. Function generator 72 has an output 74 connected to the other pole of switch 30.

Function generators 34A, 34B, 34C and 72 may be such as those manufactured by Electronic Associates, Inc., Monmouth Park Highway, West Long Beach, New Jersey.

Since the arc voltage drop versus the arc length curve changes for different gases and different current levels, and is non-linear for some combinations, a feedback network corresponding to the torch characteristics for the gas involved is used to compensate for the changing characteristics and hence maintain a relatively constant loop gain which is necessary in order to maintain adequate control in the presence of" disturbances in the system. As shown there are three function' generators 34A,B and C for three separate gases that may be admitted into gas line 16. Whatever the gas'or combination of gases used, switches 36A, B and C, or more, are closed so that the desired arc voltage drop characteristic is selected. Potentiometer 42 is set to provide a signal representative of the desired arc voltage drop. i

Assuming switch 30 is connected to the furnace, i.e., its upper closed position, and the power from source 26 is connected to the torch, transducer 28 supplies. a signal 32 proportional to the arc voltage drop. The connected function generators 34 modify the signal to compensate for the characteristics of the gas involved to maintain a relatively uniform steady state loop gain. Output signal 38 from function generator 34 is compared with the desired arc voltage drop at summing junction 40 and the difference, signal 44, provided to positionset point controller 46. Controller 46 provides an output 48 representative of the desired torch position. This signal is compared to a signal 70, from position indicator 68, which is representative of the actual torch position at set point 50 and the difference signal 52 is provided to the position controller 54. The output signal 56 of controller '54 then actuates servomechanism 58 to move torch 12 the required distance along track 8 to maintain the desired arc length. Mount travel limits 60 and 62 stop the position servo 58 when the limits of torch movement are reached.

Any change in arc voltage drop due to a disturbance is sensed by the voltage transducer 28 and the new voltage drop is sent through the system to move the torch. However, the change in arc lengththat can be accommodated in any given furnace arrangement is limited and full voltage control'may not be obtainable by this method. Thus, if too large a movement is required, an arc current control system, such as in my copending ap' plication Ser. No. 254,586 must take over to limit the current.

The control system may be modified by the use of a torch-position-arc voltage drop characteristic function generator 72 instead ofithe actual arc voltage drop. When the function generator 72 is used, switch 30 is in its lower closed position. v

The system described is for a single phase control system, but the same control system can be applied to each phase of a three-phase system to provide for individual torch control.

lclaim: 1. Apparatus for controlling the arc voltage drop in a furnace heatedby a plasma arc torch comprising a power source connected to said torch and said furnace, a voltage transducer connected to said furnace and said torch for providing a first signal representative of. the arc voltage drop, a first function. generator connected to said voltage transducer for providing asecond signal modifying said first signal toprovide a constantloop gain based on the characteristicsof the ionizing gas used, means for providing a third signal representative of the desired arc voltage drop, means for comparing said second and third signals to provide a fourth signal representative of the arc voltage drop error, and means responsive to said fourth signal for changing the distance between said torch and said furnace.

2. Apparatus according to claim 1 which includes a position indicator connected to said torch for providing a fifth signal representative of the distance between said torch and said furnace; and said means for changing the torch distanceincludes a first controller connected to said means for comparing said second and third signals for providing a sixth signal representative of the desired torch position, means connected to said position indicator and said first controller for comparing said fifth and sixth signals and providing a seventh signal representative of the torch position error, a

- second controller connected to said means for providing said seventh signal for providing a servo-position said torch and said second controller for moving said torch in response to said servo-position signal.

3. Apparatus according to claim 2 which includes means connected to said position servo-mechanism for confining the movement of said torch between a first limit towards said furnace and a second limit away from said furnace.

4. Apparatus according to claim 2 which includes a second function generator connected to said position indicator for providing an eighth signal representative of a desired torch position-arc voltage drop characteristic, and switching means a for disconnecting said torch from said voltage transducer and connecting said second function generator to said voltage transducer.

5. Apparatus according to claim 4 which includes meansconnected to said position servo-mechanism for confining the movement of said torch between a first said furnace. 

1. Apparatus for controlling the arc voltage drop in a furnace heated by a plasma arc torch comprising a power source connected to said torch and said furnace, a voltage transducer connected to said furnace and said torch for providing a first signal representative of the arc voltage drop, a first function generator connected to said voltage transducer for providing a second signal modifying said first signal to provide a constant loop gain based on the characteristics of the ionizing gas used, means for providing a third signal representative of the desired arc voltage drop, means for comparing said second and third signals to provide a fourth signal representative of the arc voltage drop error, and means responsive to said fourth signal for changing the distance between said torch and said furnace.
 2. Apparatus according to claim 1 which includes a position indicator connected to said torch for providing a fifth signal representative of the distance between said torch and said furnace; and said means for changing the torch distance includes a first controller connected to said means for comparing said second and third signals for providing a sixth signal representative of the desired torch position, means connected to said position indicator and said first controller for comparing said fifth and sixth signals and providing a seventh signal representative of the torch position error, a second controller connected to said means for providing said seventh signal for providing a servo-position signal, and a position servo-mechanism connected to said torch and said second controller for moving said torch in response to said servo-position signal.
 3. Apparatus according to claim 2 which includes means connected to said position servo-mechanism for confining the movement of said torch between a first limit towards said furnace and a second limit away from said furnace.
 4. Apparatus according to claim 2 which includes a second function generator connected to said position indicator for providing an eighth signal representative of a desired torch position-arc voltage drop characteristic, and switching means for disconnecting said torch from said voltage transducer and connecting said second function generator to said voltage transducer.
 5. Apparatus according to claim 4 which includes means connected to said position servo-mechanism for confining the movement of said torch between a first limit towards said furnace and a second limit away from said furnace. 